.NET/Java PDF, Tiff, Barcode SDK Library

Earlier in this chapter, we suggested immutability as a way to avoid having to synchronize access to your data in the .NET Framework, it s safe for any number of threads to read the same data simultaneously as long as no threads are trying to modify that data at the same time. Sometimes you may find yourself in the frustrating situation of having data that is almost read-only perhaps a website contains a message of the day which, presumably, changes only once a day, but which you may need to incorporate into each of the hundreds of web pages your busy website serves up every second of the day.

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By default, Drupal uses the duplicon for both the site logo, which appears in the web site header, and the favicon, which appears in the browser. You can upload a new logo or favicon by deselecting the Use the default logo or Use the default shortcut icon check boxes (see Figure 3-31). The logo image settings and shortcut icon settings are covered in detail in 8.

Yielding is when a thread tells the OS scheduler that it wants to give other threads a turn using the CPU now, rather than waiting to be preempted. If there are no other runnable threads, yielding does nothing, and the thread will continue to run.

The monitor s mutually exclusive locking where only one thread at a time gets to acquire a lock seems ill-suited to this scenario You could find that this shared data becomes a bottleneck all threads are taking it in turns to access it, even though that really needs to happen only once a day This is where reader/writer locks come in The idea behind a reader/writer lock is that when you acquire the lock, you declare whether you need to modify the data or just read it.

<%= Ajax.ActionLink("Click here", "GetMessage", new AjaxOptions { UpdateTargetId = "message_container", InsertionMode = InsertionMode.Replace }) %>

The lock will allow any number of threads to get a read lock simultaneously, but if a thread tries to get a write lock, it ll have to wait until all the current readers are done, and then, once a thread has a write lock, other threads attempting to get a lock of any kind will be made to wait until the write lock has been released In other words, this kind of lock supports any number of simultaneous readers, but writers get exclusive access (The precise details are, as ever, a little more complex, as a lock of this kind has to avoid the scenario where a neverending stream of readers causes a writer to wait forever New readers may be made to wait even when other readers are currently active simply to clear the decks for a waiting writer.

) While this sounds good in theory, the practical benefits can sometimes fall short of what theory might suggest You shouldn t even contemplate using a reader/writer lock unless you are seeing performance problems with a simpler monitor-based solution This kind of lock is more complex, and so it s quite possible you ll end up making things slower, particularly in cases where you weren t seeing much contention (The example we gave of a website with a message of the day is quite likely to fall into that category If the message is just a string, how long does it take to get hold of a reference to a string, really Even with hundreds of requests per second, the chances of contention are probably pretty small) It doesn t help that the first implementation offered by the NET Framework the ReaderWriterLock was, frankly, not very good.

Summary

This will render a link that displays the text Click here. When the user clicks the link, the GetMessage action will be invoked via Ajax. The response from this action (probably an HTML fragment) will be placed in an element with ID message_container. The available parameters you can pass to the AjaxOptions class to customize the behavior of the link are listed in table 12.1.

Your monitor-based solution had to be in a world of pain before ReaderWriterLock looked preferable Unfortunately, some of the problems of this lock couldn t be fixed without risking breaking existing code, so NET 35 introduced a much better replacement, ReaderWriterLockSlim If you need reader/writer locking, you should use this newer variant unless you absolutely have to support older versions of NET Be aware that unlike ReaderWriterLock, ReaderWriter LockSlim implements IDisposable, so you need to arrange for it to be disposed at the right time This means that if you use it as an implementation detail of a class, your class will probably need to implement IDisposable too..

It can sometimes be useful to abandon a loop earlier than its natural end. In the case of a foreach loop, this might mean stopping before you ve processed every item in the collection. With for or while loops, you get to write the loop condition so that you can stop under whatever conditions you like, but it can sometimes be more convenient to put the code that makes a decision to abandon a loop somewhere inside the loop body rather than in the condition. For these eventualities, C# provides the break keyword.